This invention relates to the generation of periodic pulses of applied electrical current and is more particularly directed to a method and apparatus for utilizing an alternating current power system to produce a pulsed current with a peak amperage greater than 50A.
There are many instances where the transmission of electrical current in periodic pulse form provides superior results over the continuous flow obtained with conventional systems. One example is found in the electroplating of metallic coatings on a variety of surfaces. Ordinarily, the tensile strength and ductility of the plated metal are limited to relatively low levels by the need for restricting the rate of deposit in order to prevent "burning" of the cathode by the depletion of the ions in the vicinity thereof. It has been found, however, that the rate of metal deposit can be substantially increased by passing the plating current through the electrolyte in the form of periodic pulses so that the solution equilibrium in the vicinity of the cathode is restored during the intervals between the individual pulses. The ability to plate at substantially higher rates of deposit than those ordinarily provided by continuous current power sources produces an extremely dense plate relatively free from hydrogen embrittlement. In addition, since each pulse initiates a separate nucleation site for the grain growth of the deposited metal, the undesirable columnar structure inherent in continuous current plating is virtually eliminated to produce an extremely fine-grain structure of superior resistance to stress and wear.
However, the utilization of pulsed current in the electroplating of metals has not yet met with any degree of commercial success primarily because of the relatively low amperages available at the plating cell. While it is possible to increase the power supplied by the d-c generators normally utilized in commercial plating, such solution requires complex and costly changes in the design of existing equipment in order to convert the continuous current to discrete pulses of extremely short duration. In addition, the mechanical commutators heretofore employed in the generation of pulsed current have not been able to provide the flexibility desired in the control of such parameters as the amplitude and duration of the individual pulses and the rate at which such pulses are produced.